Central
heating and hot water systems explained

A simple guide to household heating
systems

When you consider household
heating systems you should differentiate between your household heating
requirements and your domestic hot water (DHW) requirement. Whilst heating
the property is often left to the trusty old radiator (or more recently to
Under Floor Heating) there are different ways to provide domestic hot water.

It is possible to mix and match as it
were the different systems to some degree. For example, you could have a
gravity hot water system which is open vented coupled with a sealed radiator
system, or a fully open vented system where hot water and radiator circuit
are both vented (albeit separately).

This guide is intended to take a little of
the mystery out of heating systems and help you to understand your options.

There main types of
household heating and water systems include:

Conventional open vented system (including gravity fed
open vented hot water / Radiator system, or vented hot water system with
sealed primary and radiator circuit.

Combination Boiler System (Hot water on
demand with sealed radiator circuit - no cylinder or F&E tank required)

Mains Pressure (unvented) Hot Water System
- accompanied by either sealed or open vented radiator circuit.

As a brief guide only, these systems are
described in the following sections.

Fully vented / Gravity Fed System

In a fully vented system (with gravity
cold water feed to hot water cylinder and radiator circuit venting into a
small cistern - often sited in the loft) water is typically heated by a boiler
then circulated via (zone valves) two 2 port valve or a single 3 port valve
(depending on whether you have a 'Y' or 'S' plan configuration) to either:

Control of what your boiler heats is by
virtue of a programmer and the zone valves.

The term 'fully vented' comes from the fact
that no part of this type of system is pressurised. There are usually two
water tanks situated in the loft (one smaller than the other). The larger
cold water cistern (or tank) supplies water to the hot water cylinder and
does so purely by the force of gravity. The domestic hot water system (DHW) and cylinder
vent back into this cistern. The smaller 'feed and expansion' tank (F&E)
that vents the radiator circuit (and also keeps it topped up) also operates
courtesy of gravity. Both cisterns will usually be fitted with a float valve
so that a level may be maintained within the cistern and fed from a mains
supply.

A gravity system is inherently safe.
Because it should be impossible to have a dangerous pressure build-up in any
part of the system there is no risk of something bursting under pressure -
or at worst ... an explosion! That's the good news. There are however draw
backs (trade-offs) as there are with each and any system.

The main draw back with a fully vented
gravity system is the pressure you can expect from your domestic hot water.
As mentioned, the cold water feed to your hot water cylinder is under the
force of gravity. It is therefore going to be relatively low pressure.

To explain this in a
more tangible way you should consider the following:

As you elevate a vessel of water it
gains potential energy. Keeping the outlet point lets say at
ground level and raising the vessel of water, you increase the
pressure at the outlet the higher you raise the vessel. For the
purpose of this example when you raise the water vessel by approx 33
feet (10 metres) you create 1 bar of pressure out the outlet.

The pressure throughout a gravity
system will differ between outlets as the height differential
between the cistern and the outlet changes. For example, the height
difference between a cistern in a loft and an outlet on the ground
floor of a two a storey house will be greater than the height
difference of an outlet on the first floor of the same house when
fed from the same cistern. The pressure at the outlet will therefore
be greater on the ground floor than on the first floor by virtue of
the height differential.

This is why the performance of a
gravity fed shower on the first floor of a house is generally pretty
poor - especially if compared to a shower on the ground floor when
fed on the same system. The height differential between the loft
cistern and the first floor outlet (in this case the shower handset)
can be extremely small - especially if you raise the handset as high
as it will go. The nearer you go to the ceiling with it the nearer
you get to the under side of the cold water cistern.

It is important to realise that it
is the height differential between the cistern and the outlet that
makes the difference - not the size of the cistern or the amount of
water in it. The position of the hot water cylinder will
for intents and purposes not affect the pressure at the outlet by
virtue of its relative position in the system.

Providing the cold water cistern is higher
than the outlet point you have what is known as a 'positive head'. If you
attempt to create an outlet higher than the cistern you will have a
'negative head' scenario. In this situation you will not get water to flow
from the outlet under normal gravity conditions - for the simple reason that
water will not voluntarily travel up hill.

Gravity systems (open vented systems) as
mentioned are simple and safe. They may be low pressure, but they do have
the advantage of being to ONLY type of system to which you may add a booster
pump. You can even get 'negative head pumps' to cope with the problems of
outlets above cistern level.

By comparison, Water Bye Laws prohibit the
use of pumps on mains pressure systems to increase flow or pressure. Combi
systems are ains fed and will already be working flat out so attempting to
increase flow here is folly. So, if you have a low pressure gravity system
and are fed up with the poor performance of your shower you can add a
booster pump - that's the good news.

Negatives: Require loft tanks and a
hot water cylinder, so provision must be made for the space these will
require. An unpumped system will offer low pressure at taps and outlets.

Economy: Water must be heated and
stored whether or not it's used. Important to make sure your system and
storage is insulated well to minimise standing heat losses. No less
economical to produce hot water than any other system.

Combination Boilers

A Combination Boiler typically
requires no storage tanks. It provides hot water to your taps 'on demand'
by heating cold water supplied to it directly from the incoming cold water
main as required. It also provides a circuit of hot water to heat the
household radiator system which can be programmed. Combi's only heat what you
require.

Offering the advantage of space saving, Combi
Boilers are popular.

They can however have the disadvantage of
lower hot water flow rates - more often noticed when filling a bath or when
simultaneous supply is called for at more than one outlet.

Not particularly suitable for properties with
more than one bathroom or shower.

Boilers with bigger and better flow rates are
being achieved but performance is still a little lacking.

Positives: Compact unit requiring no
external tanks - good for space saving. Perform well with showers. Endless
amounts of hot water on demand albeit slowly.

Negatives: Typically lower flow rates
than other systems. Baths can take a long time to fill. Will not support
simultaneous demand from several outlets. You cannot install a shower pump to
increase performance if you discover your are less than happy.

Economy: You only heat what you need.
No storage equates to no standing losses.

This is only an advantage if you enjoy good
mains pressure within your property. If your supply pressure and flow rate
are poor there is little point.

There are different
system types that can offer this facility and you are recommended
to examine them carefully before you decide which is best for you.

On the down side, hot water at high pressure
is extremely dangerous. Unvented mains pressure hot water systems require notification to building control,
certified installation and commissioning, and annual servicing to ensure they
continue to function safely. A system storing hot water under pressure that
is incorrectly installed or poorly serviced can be extremely dangerous. There
are documented cases where unvented cylinders have malfunctioned and
exploded. To illustrate the point you might like to visit You Tube
and see a controlled experiment set up by The Myth Busters -
http://www.youtube.com/watch?v=Cv178a60Ypg&feature=related

....and this TV news article - just listen
to the damage this event caused not only to the domestic dwelling in which
this cylinder was installed but also surrounding properties. Reference is
made to the landlord 'tinkering' with the heater shortly before it
exploded!! BE WARNED !!!

Positives: Hot water at mains pressure
throughout your property. Potentially high flow rates. No feed or expansion
tanks in loft.

Negatives: Slightly more costly to
install. Some HP Systems require certification and annual maintenance for
reasons of safety. Pressure and flow only as good as your incoming main. You
may not install a shower pump on a mains-fed system.

Economy: Choosing a system type requiring annual
maintenance will lock you into long term cost. Water must be heated and
stored regardless of use.

ALTERNATIVE PRODUCTS:

Mains pressure hot water can be produced using a device known as a
Thermal Store. Thermal stores exploit the inherently safe nature of a
vented cylinder whilst producing mains pressure hot water. What is more,
they don't need a specialist installer, certification or annual servicing.
To complete the picture there are also models that will accept heat from
multiple heat sources, such as solar,
wood burner,
AGA,
ground source heat pump,
air recovery and more. They can
be an essential part of any renewable energies project.

This information is intended as a
simple guide to help explain your basic options. You are advised to seek
qualified expert advice relevant to your individual circumstances before making
any commitment. No responsibility will be accepted for matters arising from use
of this information.